Multi-fuel steam generator



I Unlted States Patent 1 1 3,543,730

[721' Inventor Earle C. Miller [56] References Cited Worcester, Massachusetts UNITED STATES PATENTS g Q A- 1 3 1 3,040,719 6/1962 Dickey 122/479 l 1 e 3,150,644 9/1964 Griffith 122/479 [45] Patented Dec.l,l970 [73] Assignee Riley Stoker Corporation OTHER REFERENCES W r t Massachusetts Discussion: Steam Generation and Utilization, ln Journal of a corporation ofMassachusens the ll'OII and Steel Institute, Aug. 1947, pg. 547- 55! TS 300 Primary Examiner-Kenneth W. Sprague Attorney-Ward, McElhannon, Brooks & Fitzpatrick ABSTRACT: Steam generators which burn fuels producing [54] S i Z STEALGENERATOR different mass flow rates for given heat production rates, inalms rawmg eluding gas recirculation arrangements for returning selected [52] [1.8. Ci 122/7, portions of exhaust gases back around to mix with fresh 122/479 products of combustion and pass with them again through a [51] Int. Cl F22g 5/06 boiler tube region to maintain a uniform mass flow rate [50] Field ofSearch.... 122/7, through the boiler tube region irrespective of changes in the fuel being burned.

Patented Dec. 1, 1970 I 3,543,730

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l Morn-rust STEAM GENERATOR This invention relates to the control of steam generation and more particularly it concerns novel arrangements which permit the use of diverse fuels together or in succession in a nor is it of constant heating value. Accordingly, in order to provide for a uniform boiler output, the boiler must be capable also of burning regular fuels, such as oil.

Different types of fuels yield products of combustion having different characteristics; and this in turn produces different effects in the boiler. For example, a byproduct fuel, such as blast furnace gas, yields a far lower quantity of heat on a per pound basis than does a conventional fuel, such as oil. Thus, in order to obtain equivalent heat outputs from the two fuels, a far greater mass flow of blast furnace gas must be passed through the boiler in a given amount of time.

Because conventional fuels such as fuel oil produce a large amount of heat for a small mass of fuel, the products of combustion are at a higher temperature, and move more slowly through the boiler, than the products of combustion of blast furnace gas having the same heat content. This presents problems in operatinga given boiler with both types of fuels. Boilers include a steam generator region and a superheater region; and the products of combustion pass successively through these two regions giving up heat to the water and/or steam passing along inside tubes which are mounted in each region. While heat transfer in both regions takes place partly by convection and partly by radiation, it happens that because the gases are-at higher temperature in the steam generator region the radiant heat transfer is greater, relative to the convective heat transfer, than it is in the superheater region. In addition, because heat transfer in the superheater region takes place primarily through convection, the superheater tubes are arranged to take maximum advantage of the mass flow of the products of combustion, since a high mass flow rate produces a greater convective heat transfer than does a low mass flow rate. I

It will be appreciated that fuel oil, whose products of combustion resultin high temperatures at low mass flow rates will transfer, on a relative basis, large amounts of heat via radiation in the steam generator region and small amounts of heat via convection in the superheater region. On the other hand,

byproduct fuels such asblast furnace gas, whose products of combustion resultin lower temperatures at high mass flow rates will transfer, on a relative basis, lesser amounts of heat in the steam generator region and larger amounts of heat in the superheater region. Because of this, it has been difficult to maintain a uniform output from a steam boiler in which diverse fuels are used.

Another problem which has arisen in connection with at tempts to accommodate steamboiler systems to different types of fuel, has been that of handling the widely disparate gas flows which accompany the burning of the different fuels. In the past, separate fans and separate exhaust systems were required. However, this was expensive, and furthermore it did not lend itself rapidly to rapid changeover from one type of fuel to the other since time was required to bring each fan up to operating speed.

The present invention overcomes all of the above described difficulties and provides a boiler arrangement which is quite simple in construction and easy to control. Moreover, the novel boiler arrangement of the present invention is capable of accommodating different types of fuel and of rapidly shifting from one type to the other without affecting the output. In addition, the present invention makes it possible to maintain a substantially uniform steam temperature even when the load on the boiler varies. This last feature is particularly significant for it allows the boiler to operate close maximum output steam temperature for efficient operation and yet avoids the danger of breakdown which can occurifthis maximum temperature is exceeded.

The above andother features of the present invention are attained by'means of a novel gas flow control arrangement. According to this arrangement, the hot gases; which pass through the steam generator region and the superheater region are directed through a fan and then past a selection device, such as a damper, which redirects a selected portion of these gases back toward the combustion chamber where they mix with fresh products of combustion from the combustion chamber and pass along with them back through the boiler. This increases the mass rate of flow through the combustion chamber and also increases the convective heat transfer in the superheater region of the boiler. The damper is made adjustable to control the percentage of gases which are recirculated. Thus, whenever the furnace is switched from one fuel to another, the difference in mass rates of flow of the combustion gases is quickly compensated for by adjustment of the damper.

An exhaust fan is interposed in the path of combined gas flow, preferably between the boiler and the damper. This fan may be of essentially constant capacity. This is possible. even though fuels producing different mass flow rates are burned at different times, beca use the recirculation arrangement and the adjustable damper will maintain a uniform mass flow rate through both the boiler and the fan irrespective of changes in the mass flow rates produced by the different fuels being burned.

Further specific features and advantages of the invention will be hereinafter more fully set forth with reference to the annexed drawings, showing a presently preferred embodiment of the invention and certain modifications thereof, in which:

FIG. 1 is a semischematic illustrating a steam generating and control system in which the present. invention is embodied; and

FIG. 2 is a graph illustrating the operation of the system of FIG. 1.

The steam generating system of FIG. 1 includes a furnace 10, the outer boundaries of which are defined by walls 12. The walls 12 are of typical construction and may comprise an outer steel shell lined internally with refractory material.

The lower portion of the furnace 10 is constructed to form a combustion chamber 14. A pair of indentations or abutments 16 define the upper limit of the combustion chamber 14 and form a throat 18 through which gaseous products of cornbustion pass in an upward direction. Means are provided for burning in the combustion chamber 14, different typesof fuel'whose products of combustion have different masses for given heat contents. Thus, there is provided a blast furnace gas supply means comprising a pair of gas supply ducts 20 which have openings 22through a lower face 24 of the abutments 16. Blast furnace gas, which is characterized by combustion products having a relatively large mass for a given heat content, is supplied from an external source (not shown) and passes through the gas supply ducts 20, to the combustion chamber 14. The flow rate of this blast furnace gas is controlledby dampers 26 arranged in the ducts 20. The dampers'26 are operated by hydraulic actuators There are additionally provided oil'fuel supply means comprising. a pair of oil supply conduits 30 arranged to open into the combustion chamber 14 at locations in the abutments '16 which are concentric with the gas supply ducts 20. Oil, which produces combustion products having a relatively small mass for a given heat content is supplied from an external source (not shown) and passes through a common feed line 32 to the supply conduits 30 and from there to the combustion chamber 14. The oil flow rate is controlled by a valve 33 interposed in the common feed line 32. The valve 33 is operated by a hydraulic actuator 34.

Combustion air is supplied to the furnace 10 by means ofan air supply fan 36. The output of the fan 36 is directed through an air supply conduit 38. The conduit 38' doubles back on itself and extends immediately adjacent an outer surface of the furnace If) so that the air passing through the conduit will be preheated as it passes toward the combustion chamber 14. The conduit 38 opens into an annular jacket 46 which surrounds the combustion chamber. This provides an insulative effect for this portion of the furnace and at the same time it permits recovery by the combustion air ofa portion of the furnace heat which might otherwise be dissipated. The air which has moved through the jacket 40 passes into the combustion chamber Min the vicinity of the gas supply ducts 20 and the oil supply conduits 30. The air is mixed with the fuel being supplied from one or both ofthese sources and the resulting combustible mixture is burned within the combustion chamber.

The portion of the furnace immediately above the combustion chamber 14 is formed to define a boiler tube region 42. Within the boiler tube region are nests of boiler tubes 44 through which water and steam pass. The fluid in these tubes receives heat from the gaseous products of combustion as these gases pass up from the combustion chamber 14 through the throat 18 and into the boiler tube region 42.

The specific arrangement of the boiler tubes M may vary depending upon the size of the system and its particular requirements. However, the present system, like most boiler arrangements, includes steam generation tubes 46 and superheater tubes 48. The steam generation tubes 36 line the walls of the boiler tube region 42, and they also extend down into the combustion chamber 14. The steam generation tubes 46 are connected to upper and lower steam drums 50 and 52 in a manner which permits continuous recirculation of water through the furnace while it accumulates heat.

The upper steam drum t functions as a steam separator and it is in this drum that the actual vapor phase is separated from the liquid phase of the fluid in the system. The vapors, however, do contain certain amount of moisture in the liquid phase. This liquid moisture, which is carried along as drops or mist, is unsuitable for most steam utilization devices such as turbines. in order to remove this moisture and to superheat the steam, the steam is passed through the superheater tubes 48. The superheater tubes 48 are arranged somewhat downstream of the steam generator tubes 46', and the hot gaseous products of combustion which emanate from the combustion chamber 14 first'pass by the steam generator tubes 4-6; and thereafter they pass over the superheater tubes 48. Beyond the boiler tube region 42 there is formed a gas outlet conduit 54 through which the hot gaseous products of combustion pass after they leave the boiler tube region. In the conduit 54 there are positioned economizer tubes 56 through which boiler feedwater passes. This feedwater becomes preheated in the economizer tubes by receiving some of the heat remaining in the hot combustion gases which have left the boiler tube region 42. Beyond the economizer tubes 56 the conduit 54 directs the combustion gases to pass close to the incoming air in the air supply conduit 38 to heat it.

The hot gas outlet conduit 52 extends to the inlet of an outlet fan 57. The fan 57 is of substantial size and is built for generally constant volume operation.

The outlet of the fan 5'7 opens into a junction 58 which leads to an exhaust stack 60 and to a return conduit 62. The exhaust stack 60 permits exhaustion of a portion of the spent hot gases. The return conduit 62 leads back down to a return chamber 64 surrounding the upper surface of the abutments 16. Openings 66 are formed in these surfaces and gases which pass through the return conduit enter through the openings 66 and mix with the hot combustion gases from the combustion chamber.

A recirculation damper. 68 is provided in the junction 58, and this damper is arranged to divide the output gases from the fan 57 between the stack of) and the recirculation conduit 62. The damper 6b is adjustable in the junction 58 to control the ratio of gases directed to the stack an and to the recirculation conduit 62. A hydraulic actuator 70 is arranged to control the recirculation damper adjustments.

The hydraulic actuators are each controlled by associated hydraulic valves "72 interposed in hydraulic lines '74. These hydraulic lineslead to a common pump '76 and hydraulic reservoir '76. The valves '72 are in turn operated by solenoids fill which receive signals via electrical lines from a master control 841. The master control in turn receives signals indicau ing load condition, steam temperature, input air flow, etc.; and based upon these items it adjusts the'electrical and hydraulic elements of the system to control air and fuel flow to the furnace. Steam temperature is measured by means of a thermocouple 86 positioned in a manifold 88 to which the superheater tubes 48 are connected. The signals produced by the thermocouple 86 are transmitted via a line Ml to the master control 84. Input air flow is measured by means of pressure detectors 92 positioned in the vicinity of a constriction 94 in the air supply conduit 38. Electrical signals representative of air flow are produced in these detectors and these signals are transmitted via lines 96 to the master control 845. The master control 84- also produces signals on lines 93 to control solenoids wil for operating a hydraulic valve 102 which in turn controls an actuator 10 for moving a damper ms in the supply conduit 38. This damper in turn controls the flow of air through the conduit and in this manner a proper air-fuel mixture can be maintained und'erall operating conditions.

During operation of the system, fuel is burned in the combustion chamber 14 and the hot gaseous products of com bustion thus formed pass upwardly through the throat it and into the boiler tube region in the boiler tube region, the hot gases pass by and transmit heat first to the steam generator tubes 46 and then to the supcrheater tubes 48. Thegases then pass through the gas outlet conduit 54 and over the economizer tubes 56 to the outlet fan 57. The fan 57 maintains movement of the gases from the combustion chamber 14 through the boiler tube region 42. Beyond the fan 57 the gaseous products of combustion pass into the junction 5:; where they encounter the recirculation damper 68. Depending upon the damper setting a first portion of these gases will pass out through the exhaust stack while the remaining portion will be returned via the return conduit 62 to the return conduit 64. This remaining portion then passes through the openings 64 back into the boiler tube region 42. where it mixes with and adds to the'fresh gaseous products of combustion which are passing up through the throat 18 from the combustion chamber 14.

The heat transfer from the hot gases to the fluid in the tubes of the boiler tube region 42 takes place in two ways. The initial heat transfer occurs through the steam generator tubes 46. The heat transfer in this instance occurs primarily through radiation. Thereafter, the gases encounter the supcrheater tubes 48 and in this instance heat transfer occurs primarily through convection.

The radiative heat transfer which occurs closer to the com bustion chamber 14 depends for its effectiveness upon the temperature and radiation characteristics of the burning gases. It does not depend upon the mass or the velocity of the gases. On the other hand, the convective heattransfer which occurs further from the combustion chamber 1,4 and through the superheater tubes d8 depends, for its effectiveness, upon the mass rate of flow of the hot gases. The radiation characteristics and temperature of the gases are of less import in this portion of the boiler tube region.

Now for a given rate of heat production, a different mass flow rate of gaseous combustion products is produced for each of the two types of fuel which are burned in the furnace llfil. When oil is burned at a given heat production rate a relatively small mass flow is produced. On the other hand, when blast furnace gas is burned at the same heat production rate a much larger mass flow is produced. These different mass flow rates have a rather minor effect on the radiative heat transfer through the boiler tubes 44. However, they do have a substantial effect upon the convective heat transfer to the fluid in the superheater tubes 43. As a result, for a given rate of heat production a different output steam temperature would be produced for different fuels. This effect is illustrated in the diagram of HG. where output steam temperature is plotted against load, or heat production, for oil and for blast furnace gas.

The gas recirculation arrangement of the present invention makes it possible to maintain a substantially constant output steam temperature independently of load and independently of the type of fuel being burned.

This is achieved by returning a selected portion of the exhaust gases via the return conduit 62 to the boiler tube region 42 where they mix with and augment the gases passing into the boiler tube region from the combustion chamber 14. When the fuel being burned is oil, which produces a lower mass flow rate, the recirculation damper 68 is adjusted to return a maximum percentage of the exhaust gases. This will greatly increase the mass flow rate of the gases passing over the superheater tubes; and even though the temperature of the fresh combustion gases from the combustion chamber 14 is lowered somewhat by the temperature dilution effects of the recirculated gases, at greater amount-of heat will be transferred to the steam in the superheater tubes 48 because of the greatly increased effectiveness of the convective heat transfer obtained by the increased mass flow rate. I

The furnace and boiler size capacities are established so that, as shown in FIG. 2, where blast furnace gas is burned at 100 percent load the steam produced will be at operating temperature with no recirculation. However, as the load decreases, say to 50 percent greater amounts of recirculation are obtainedby operation of the recirculation damper 68 so that the operating temperature will be maintained. The distance (a) in the diagram of FIG. 2 illustrates the amount of. temperature augmentation that is produced in this manner. Now where oil fuel is burned, a certain amount of gas recirculation is required even at 100 percent load to increase the output steam temperature from that which would be obtained without recirculation, to normal operating temperature. This temperature difference is indicated by the distance (b) in HQ. 2. As the load'decreases the amount of gas recirculation is increased so that at 50 percent load, using fuel oil, the amount of recirculation needed to maintain operating steam temperaturewould be that which will produce the temperature difference indicated by the distance (c) in FIG. 2.

It will be appreciated that combinations of fuel can be burned in the furnace and-these will produce the temperature-load characteristics indicated by the intermediate line in FIG. 2. Corresponding amounts of recirculation would then be required to maintain the operating steam output temperature for different load conditions.

A feature of the present invention resides in the fact that the changes in recirculation are achieved entirely by means of the recirculation damper 68. Thus, these changes can be made very rapidly to accommodate changes in load and even changesfrom'one type of fuel to the other. Thus, for example if at 50 percent load, it were'desired to switch from oil to blast furnace gas, this could be done by operating the associated hydraulic actuators 28 and 34 controlling the flow of these fuels and then adjusting the recirculation damper 68. It is not necessary to adjust the outlet fan 57 and in fact this fan may be of constant capacity, since the flow through the fan is not affected by changes in the amount of gas recirculation.

I claim: t

l. A method of controlling steam temperature in a furnace boiler arrangement wherein fuels having different mass flow ratesfor given heat production rates are burned, said method comprising the steps of passing gaseous products of combustion of said fuels first through a first region containing boiler tubes which receive heat from said products primarily by means of radiant heat transfer, then passing the gaseous products through. a second region containing superheater tubes which receive heat from said products primarily by means of convective heat transfer and adjustingthe mass rate of flow of products of combustion through said second region by recirculation therethrough of selected portions of said products of combustion, said recirculation being achieved by passing selected portions of the products of combustion which have passed out from said second region back around to be mixed with fresh products of combustion prior to their entry into said region, and said selected portions being selected in a manner such as to maintaina substantially constant mass flow rate through said second region. a

2. A method according to claim 1 wherein said selected portions are selected by adjustment of damper means in the path of said products of combustion beyond said second region.

3. in a multifuel steam generating system the combination comprising a furnace including means defining a combustion chamber, first fuel supply means constructed and arranged to inject, for combustion in said chamber, a fuel characterized by products of combustion having a low mass flow rate for a given heat production rate, second fuel supply means constructed and arranged to inject, for combustion in said chamber, a fuel characterized by products of combustion having a high mass flow rate for a given heat production rate, means defining a boiler tube region containing boiler tubes and located adjacent said combustion chamber whereby boiler tubes in said region receive heat from the products of combustion generated in said combustion chamber, first gas conduit means arranged to direct the products of combustion which have passed over said boiler tubes out from said boiler tube region, fan means arranged in said first gas conduit means, second gas conduit means connected to said first gas conduit means downstream of said fan, said second gas conduit means extending back to said. boiler tube region for directing a selected portion of the products of combustion from said first conduit means back through said boiler tube region and across said boiler tubes and partition means arranged within the junction between said first and second gas conduit means, said partition means being adjustable in accordance with the selective operation of said fuel supply means to maintain'a constant mass rate of flow of products of combustion through said boiler tube region.

4. A combination according to claim 3 wherein said first fuel supply means is constructed to supply a liquid fuel to said chamber and said second fuel supply means is constructed to supply a gaseous fuel to said chamber.

5. A combination according to claim 3 wherein said partition means is a damper arranged to divide gas flow from said boiler tube region between an'exhaust stack and said second conduit means.

6. A combination according to claim 3 wherein said combustion chamber communicates with said boiler tube region via a throat and wherein said throat is surrounded by a chamber connected to said second conduit, said second chamber being-formed with openings leading into said throat.

7. A combination according to claim 6 wherein said throat is formed by abutments protruding inwardly of the region between said combustion chamber and said boiler tube region.

8. A combination according to claim 7 wherein said fuel supply means extend through lower facing walls of said abutments toward said combustion chamber and said openings from the chamber connected to said second conduit extend through upper facing walls of said abutments toward said boiler tube region.

9. in .a steam generating system a furnace including a combustion chamber and fuel supply means, said combustion chamber and fuel supply means being adjustable to burn fuels having different mass rates of flow of products of combustion for given heat production rates, a boiler tube chamber arranged adjacent said combustion chamber, gas conduit means arranged to direct products of combustion from said combustion chamber through said boiler tube chamber, recirculation conduit means arranged to direct selected portions of the gases exiting from said boiler tube chamber back around to mix with fresh products of combustion from said combustion chamber and reenter said boiler tube chamber and means arranged to control said selected portions in accordance with said fuel supply means to maintain a substantially uniform mass flow rate through said boiler tube region.

it). A combination according, to claim 3*- wherein said boiler tube region includes boiler tubes and superheater tubes, said boiler tubes being arranged to receive heat primarily through means arranged in the path of combined flow of fresh products of combustion and recirculated gases.

13. A combination according to claim 12 and including control means arranged in conjunction with said fuel supply means for maintaining a substantially uniform mass rate of flow through said fan means irrespective of adjustments of said fuel supply means. 

